Tension Control System for Deformable Nip Rollers

ABSTRACT

A method and apparatus for controlling web tension in a web printing press having a plurality of sets of deformable nip rollers, each set driven by a separate driver, the web passing sequentially through each set of deformable nip rollers and forming a web segment therebetween. The driver of a downstream set of deformable nip rollers is adjusted to set the tension of a first web segment and a controller adjusts a first characteristic of the driver of the upstream set of deformable nip rollers to maintain the tension of a second web segment constant. Alternatively, a sensor measures a second characteristic of the second web segment and the controller adjusts the first characteristic of the driver of the upstream set of deformable nip rollers based on the measurement from the sensor.

BACKGROUND OF THE INVENTION

The present invention relates generally to web printing presses and moreparticularly to a method and apparatus for controlling the tension ofthe web in a printing press including two or more sets of nip rollerswith deformable covers.

Web printing presses print a continuous web of material, such as paper.The web travels through nips formed by opposing nip rollers. The web ismoved on its way by driven nip rollers which are driven by respectivenip roller drivers.

Tension in the web must be maintained within a desired range in order toachieve smooth operation of the printing press. At the same time, thevelocity of the web, and hence the rotational speed of nip rollers, mustbe held relatively constant to achieve good print product quality.Tension in a web span between two nips can be adjusted by controllingthe speed of the nip roller driver in the downstream nip.

Multiple types of nip rollers have been used in conventional webprinting presses. Some nip rollers were formed with a deformable surfacelayer such as urethane. Nip rollers with a deformable surface were foundto provide less than ideal operation because of uneven ribbon-to-ribbontension in upstream web spans.

As a result, nip rollers formed with a deformable surface such asmicrocellular foamed urethane were introduced (see, U.S. PatentPublication No. 2006/0157924 A1 published on Jul. 20, 2006 andincorporated herein by reference). Nip rollers with this type ofdeformable surface layer were found to overcome the unevenribbon-to-ribbon tension problem of the earlier nip rollers. However, itwas found that any nips with non-rigid covers have an effective gainthat is a function of the tension difference across the nip roller, sothat if the tension of the web downstream from the nip roller ischanged, the tension of the upstream web will also change requiring anadditional tension adjustment.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and apparatuswhich allows for adjustment of the tension of a downstream span of webwithout affecting the tension of an upstream span of web.

The present invention provides a method for controlling web tension in aweb printing press having a plurality of sets of deformable nip rollers,each set of deformable nip rollers driven by a separate driver, the webpassing sequentially through each set of deformable nip rollers, eachadjacent set of deformable nip rollers forming a web segmenttherebetween. A first characteristic of the driver of a downstream setof deformable nip rollers is adjusted to set the tension of a first websegment which is between the downstream set of deformable nip rollersand an upstream set of deformable nip rollers. Then, a firstcharacteristic of the driver of the upstream set of deformable niprollers is adjusted to maintain a constant tension on a second websegment which is immediately preceding the upstream set of deformablenip rollers.

In an alternative embodiment, a second characteristic of the second websegment is measured and the first characteristic of the driver of theupstream set of deformable nip rollers is adjusted based on themeasurement of the second characteristic.

The present invention also provides an apparatus for controlling webtension in a web printing press having a plurality of sets of deformablenip rollers, each set of deformable nip rollers driven by a separatedriver, the web passing sequentially through each set of deformable niprollers, each adjacent set of deformable nip rollers forming a websegment therebetween. The apparatus includes a circuit for adjusting afirst characteristic of the driver of a downstream set of deformable niprollers to set the tension of a first web segment which is between thedownstream set of deformable nip rollers and an upstream set ofdeformable nip rollers. The apparatus also includes a controller foradjusting a first characteristic of the driver of the upstream set ofdeformable nip rollers to maintain a constant tension on a second websegment which is immediately preceding the upstream set of deformablenip rollers.

In an alternative embodiment, the apparatus also includes a sensor formeasuring a second characteristic of the second web segment and thecontroller adjusts the first characteristic of the driver of theupstream set of deformable nip rollers based on the measurement from thesensor.

The first characteristic may be the rotational speed of the driver. Thesecond characteristic may be the tension of the second web segment, thespeed of the second web segment, or the amount of torque in the driverof a set of deformable nip rollers immediately preceding the upstreamset of deformable nip rollers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and related objects, features and advantages of the presentinvention will be more fully understood by reference to the followingdetailed description of the presently preferred, albeit illustrative,embodiments of the present invention when taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a block diagram of an embodiment of the present inventionhaving closed loop control based on a feedback signal provided from atension sensor;

FIG. 2 is a block diagram of an embodiment of the present inventionhaving closed loop control based on a feedback signal provided from avelocity sensor;

FIG. 3 is a block diagram of an embodiment of the present inventionhaving closed loop control based on a feedback signal provided from atorque sensor; and

FIG. 4 is a block diagram of an alternative embodiment of the presentinvention having open-loop control.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows a block diagram of an apparatus for setting the tension ofa downstream web without affecting the tension of an upstream web in aweb printing press 10 according to one presently preferred embodiment ofthe invention. Web 12 is moved by driven nips 22, 28 and 34 in thedirection indicated by arrow 14. Nips 22, 28 and 34 are formed by niprollers 20 and 24, 26 and 30, and 32 and 36, respectively. Nip rollers22 and 24, 26 and 30, and 32 and 36 rotate as indicated by arrows 27 andare formed with a deformable surface as described above and in moredetail in U.S. Patent Publication No. 2006/0157924 A1.

Nip rollers 20, 26 and 32 in FIG. 1 are driven by nip roller drivers 38,40 and 39, respectively. Nip roller drivers 38, 40 and 39 may be, forexample, electric motors, or other type of suitable drivers. Drivecontrollers 48, 50 and 49 provide control signals to nip roller drivers38, 40 and 39, respectively, to control the rotational speed ofrespective nip rollers 20, 26 and 32. The control signals are providedfrom controller 70 to the drive controllers 48, 50 and 49 on respectivesignal lines 58, 60 and 59, respectively.

Upstream web span 16 is formed between nips 22 and 28, while downstreamweb span 18 is formed between nips 28 and 34. As one of ordinary skillin the art will readily recognize, additional nips may be provided inthe printing press 10 within the context of the present invention, andthe principles of this invention apply in that event as well. A tensionsensor 92 directly measures the tension in upstream web span 16 andprovides a corresponding output signal on line 93 to controller 70.Controller 70 also includes an input 60 which is used by an operator toset the tension of downstream web span 18 according to principles of theinvention.

In the embodiment of FIG. 1, closed loop control is used to maintain thetension of upstream web span 16 at a constant value when an operatordesires to change the tension of the downstream web span 18. Inparticular, when an operator inputs a tension adjustment for upstreamweb span 18 to controller 70 via input 80, controller 70 firstcalculates an updated value for the control signal 59 so that drivecontroller 49 will adjust the rotational speed of nip roller driver 39thereby changing the tension of downstream web span 18 to the desiredvalue. Since the change in tension of downstream web span 18 would alsoresult a change in the upstream web span 16 when using deformable niproller, controller 70 also changes the rotational speed of nip rollerdriver 40 in a manner that will maintain the tension of upstream webspan 16 at a constant value. This requires a decrease in the rotationalspeed of nip roller driver 40 when the tension of downstream web span 18is increased (by increasing the speed of nip roller driver 39), and anincrease in the rotational speed of nip roller driver 40 when thetension of downstream web span 18 is decreased (by decreasing the speedof nip roller driver 39). In particular, controller 70 receives a signalon line 93 from tension sensor 92, and compares the received tensionsignal with the desired tension level and adjusts, in a closed loopmanner, the control signal 60 so that the result of the comparisonbetween the received tension signal and the desired tension level iszero. In this manner, whenever an operator changes the tension ofdownstream web 18, the tension of upstream web 16 will be automaticallykept constant by closed loop control via the feedback signal providedfrom tension sensor 92. By maintaining the tension on upstream web span16 at a constant level, the change of tension on downstream web span 18is invisible to all preceding nips and spans.

Although the embodiment of FIG. 1 shows a single controller whichseparately sets and controls the speed of nip roller drivers 38, 39 and40, as one of ordinary skill in the art will readily recognize, each niproller driver 38, 39 and 40 could be separately controlled, and that theclosed-loop control is only applied in this illustrative embodiment tocontrol the nip roller driver which is upstream from the web span havingits tension changed. Of course, as one of ordinary skill in the art willreadily recognize, the techniques of this invention can be applied toallow adjustment of any span in a printing machine without affecting thetension of any other span. Thus, in the embodiment of FIG. 1, nip rollerdrives 38 and 39 are shown being set by controller 70, but couldalternatively could be set by other means known to one of ordinary skillin the art. Also, although FIG. 1 shows a controller 70 which comparesthe signal from tension sensor 92 to a desired value to calculate thefeedback control signal 60 to apply to drive controller 50, as one ofskill in the art would readily recognize, this could alternatively bedone in the analog domain using a comparator and other ancillarycircuitry, where the comparator compares an analog signal from thetension sensor 92 to a predetermined value representing the desiredtension setting. Finally, although FIG. 1 shows drive controllers 48, 49and 50 as separate from controller 70, as one of ordinary skill in theart will understand, the drive controller functions could alternativelybe implemented either within controller 70 or separately within niproller drivers 38, 39 and 40.

FIG. 2 shows a first alternative embodiment of the present inventionwhich also relies on closed loop control to maintain the tension ofupstream web span 16 at a constant value when the tension of downstreamweb span 18 is intentionally changed by the operator. The only changefrom the embodiment of FIG. 1 is that a velocity sensor 95 is providedto monitor the velocity of upstream web span 16 and provide acorresponding velocity signal 98 to controller 70 (and tension sensor 92is omitted). Since the velocity of the web span is proportional to thetension thereof, controller 70 compares the measured velocity signal 98to the desired velocity value and generates a control signal 60 for thedrive controller 50 connected to nip roller driver 40 which, because ofthe closed loop nature of the system, will cause the difference betweenthe measured velocity signal 98 and the desired velocity value toquickly become zero, even after the tension on downstream web span 18 ischanged.

FIG. 3 shows a second alternative embodiment of the present inventionwhich also relies on closed loop control to maintain the tension ofupstream web span 16 at a constant value when the tension of downstreamweb span 18 is intentionally changed by the operator. The only changefrom the embodiment of FIG. 1 is that a torque sensor 90 is provided innip roller driver 38 to provide a torque signal 57 to controller 70 (andtension sensor 92 is omitted). Since the torque of the nip roller driver38 is proportional to the tension on upstream web span 16, controller 70compares the measured torque signal 57 to a toque value that correspondsto the desired tension on upstream web span 16 and generates a controlsignal 60 for the drive controller 50 connected to nip roller driver 40which, because of the closed loop nature of the system, will cause thedifference between the measured torque signal 57 and the desired torquevalue to quickly become zero, even after a change on the tension ondownstream web span 18.

FIG. 4 shows a third alternative embodiment of the present inventionwhich, unlike the other embodiments, does not rely on closed loopcontrol to maintain the tension of upstream web span 16 and thus doesnot require any sensor to monitor the tension of upstream web span 16.Instead, when the operator inputs a tension change for downstream webspan 18, controller 70 automatically calculates the amount of changenecessary for both control signal 59 and control signal 60 based oncharacteristics stored in memory. These characteristics can be definedempirically or using predictive models, as understood by one of ordinaryskill in the art, and, for example, control signal 60 could be a fixedpercentage of control signal 59.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments and various aspects thereof,it will be appreciated by those of ordinary skill in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the invention. It is intended that the appendedclaims be interpreted as including the embodiments described herein, thealternatives mentioned above, and all equivalents thereto.

1. A method for controlling web tension in a web printing press having aplurality of sets of deformable nip rollers, each set of deformable niprollers driven by a separate driver, the web passing sequentiallythrough each set of deformable nip rollers, each adjacent set ofdeformable nip rollers forming a web segment therebetween, comprisingthe steps of: adjusting a first characteristic of the driver of adownstream set of deformable nip rollers to set the tension of a firstweb segment which is between the downstream set of deformable niprollers and an upstream set of deformable nip rollers; and adjusting afirst characteristic of the driver of the upstream set of deformable niprollers to maintain a constant tension on a second web segment which isimmediately preceding the upstream set of deformable nip rollers.
 2. Themethod of claim 1, further comprising the step of: measuring a secondcharacteristic of the second web segment; and wherein the step ofadjusting a first characteristic of the driver of the upstream set ofdeformable nip rollers is based on the measurement of the secondcharacteristic.
 3. The method of claim 2, wherein the firstcharacteristic is the rotational speed of the driver.
 4. The method ofclaim 2, wherein the second characteristic is the tension of the secondweb segment.
 5. The method of claim 2, wherein the second characteristicis the speed of the second web segment.
 6. The method of claim 2,wherein the second characteristic is the amount of torque in the driverof a set of deformable nip rollers immediately preceding the upstreamset of deformable nip rollers.
 7. An apparatus for controlling webtension in a web printing press having a plurality of sets of deformablenip rollers, each set of deformable nip rollers driven by a separatedriver, the web passing sequentially through each set of deformable niprollers, each adjacent set of deformable nip rollers forming a websegment therebetween, comprising: means for adjusting a firstcharacteristic of the driver of a downstream set of deformable niprollers to set the tension of a first web segment which is between thedownstream set of deformable nip rollers and an upstream set ofdeformable nip rollers; and a controller for adjusting a firstcharacteristic of the driver of the upstream set of deformable niprollers to maintain a constant tension on a second web segment which isimmediately preceding the upstream set of deformable nip rollers.
 8. Theapparatus of claim 7, further comprising: a sensor for measuring asecond characteristic of the second web segment; and wherein thecontroller adjusts the first characteristic of the driver of theupstream set of deformable nip rollers based on the measurement from thesensor.
 9. The apparatus of claim 7, wherein the first characteristic isthe rotational speed of the driver.
 10. The apparatus of claim 8,wherein the sensor measures the tension of the second web segment. 11.The apparatus of claim 8, wherein the sensor measures the speed of thesecond web segment.
 12. The apparatus of claim 8, wherein the sensormeasures the torque in the driver of a set of deformable nip rollersimmediately preceding the upstream set of deformable nip rollers.